text:
Why would a pilot request an unusual approach/runway to ATC?
In another SE Aviation question about the preferred approach for commercial aviation (Visual vs ILS) there's the following useful explanation:

In most ATC facilities, the Tower decides what runway to use, and the
  approach control calls the type of approach generally on offer(this
  can vary depending on facilities and how they agree to run things). A
  pilot can always request a different approach... they might be
  delayed, sometimes significantly if they insist on something way out
  of the loop

I'm curious what would be examples of a pilot insisting on "something way out
of the loop"?
What's the preferred approach for a passenger jet?

Interest only: Here is an only slightly out of the loop real world event - Jean Batten International aka Auckland Airport in NZ has an over-land or over water approach with the former usually being used due to prevailing wind. When the US President visited NZ "some while ago" they used the "over water" approach for Airforce 1 - presumably to minimise the opportunities for the bad guys to do something undesirable.  I don't know which direction was used on departure.

Great Barrier Airlines serves Islands in the Hauraki Gulf. Some years ago their fleet included 4 x  3-engined STOL capable Britten-Norman Trislanders.   I noted on a number of occasions at Jean Batter airport that these used to take off towards their destination (towards landward) regardless of what everyone else was doing on a given day, and would alter course soon after takeoff when all "normal" airliners were still climbing out.
Britten-Norman Trislander

For currency, airline pilots have to do various approaches every so often to remain current or for the plane to be current (RNAV/ILS Cat II/III approaches). So, every now and then when coming to another airport they could request to shoot that approach instead of the one being used. Sometimes it's a non event (no traffic, or the few other aircraft either fit well before or behind them); other times, it could hinder the flow of operations if they're allowed to do full or certain procedures (various circling to land procedures).
Another very common request that falls in this category is to land or depart in the opposite direction of what's being advertised. This in recent years has become more cumbersome for controllers (due to more and new rules implemented), so it's being used less often.
Sometimes when there's little traffic it's fairly easy to accomplish this kind of arrival or departure. Other times, it becomes a nightmare, and unless it's operationally mandated the pilot will be told, unable opposite direction, due to traffic. When it's operationally needed, then it will be told to hold somewhere to wait until there's space to get them out for departure; or vectored around or put in holding if needing to land opposite direction, and they can be accommodated.

another example is when the pilot requests to fly the full approach, rather than vectors-to-final

Here is what happens when the crew does not ask for Cat II/III and tries to do it anyway.

@JanHudec I tried reading that accident report a couple of times & there's even a very nice Youtube Video of the incident but I'm still not getting what exactly went wrong there. Could someone elaborate?

@curious_cat Basically, when Cat II/III procedures are in effect the runway and critical areas are more protected, to prevent signal disruption from aircraft or vehicles blocking the signal when doing an auto-land approach. When you want to do an autoland for currency, you often won't get those areas protect, so you have to be more vigilant or willing to turn off the auto-pilot or go around if you notice the signal is getting weird.

@curious_cat, specifically in this case it was apparently the departing plane that was in the air, but still over the runway (because visual environment allows less separation) that disrupted the localizer signal and confused the autopilot during roll-out.

At times ATC wants arrivals to land even though the runway-in-use has a tailwind component. This may occur e.g. during a weather front passage. However, some aircraft types may not be certified for more than a minor tailwind. Also company procedures or pilot's personal minimums may apply. If the runway is wet, snow-covered or of the shorter variety even less tailwind is acceptable. I once listened to an ATC conversation where ATC wanted an airliner to land in a 10-15 knots tailwind. The pilot in command refused, referring to company policy.
For smaller aircraft, strong crosswind may be the triggering factor which prompts the pilot to request a different runway. Especially if he flies a taildragger aircraft.
Modern airliners typically can handle quite strong crosswinds, but not if the runway is wet or otherwise slippery. In those cases a request for a different runway may also be the right choice.
Requesting a different kind of approach may occur because the pilot wants to maintain his proficiency to fly that particular type of approach. Sometimes the reason is to save time, such as when requesting a close-in visual approach instead of adhering to the default ILS approach.


text:
What is the meaning of "Docile pitch behavior" in the context of Pugachev's Cobra?
In another answer on SE Aviation it is mentioned that the ability to perform  Pugachev's Cobra  in a non-thrust-vectored aircraft needs "docile pitch behavior of the airframe up to approx. 110° angle of attack"

Can an F-16 perform Pugachev's Cobra?
What does "docile pitch behavior" mean in this context? Does it mean that when the control surfaces are returned to neutral the aircraft has a natural pitch down moment that returns it to its horizontal position?
If so, does the pilot actually have control over how long he can hold the vertical position in Pugachev's Cobra? What control surface enables him to exercise this authority in the post-stall regime when I'd have assumed the
control surfaces lack such authority?
Or is the holding time of the vertical attitude in Pugachev's Cobra beyond the pilot's control and only related to some sort of "natural period" of this maneuver in that aircraft linked to how long it takes the restoring moment to return the aircraft to its nose down attitude?

"Docile" means no big pitching moments over the angle of attack range. Since the control power with fully stalled control surfaces is rather small, the airframe must not create strong pitching moments, but keep a small pitch-down moment so the airplane returns eventually to normal angles of attack which allow regular flight.
The maneuver is dynamic, so the pilot needs to build up a specific pitch rate. Inertia will keep the aircraft pitching up, and the small pitch-down moment (and aerodynamic damping) will reduce the pitch rate so that the pitch-up stops when the nose is pointing straight up. What follows is the consequence of the small pitch-down moment: The aircraft slowly returns to controlled flight, and once control authority is re-established, the pilot stops the pitching motion.
By selecting the initial pitch rate, the pilot can control the maximum angle of attack, but once he leaves the flight regime of attached flow, he has to wait until inertia and aerodynamics do their part.

Thanks! Now I understand it better. So the max cobra duration is essentially proportional to whatever max initial pitch rate the pilot can achieve to enter the manouever? i.e. The greater the initial pitch rate the longer it takes the damping and restoring pitch-down moment to return the aircraft to controlled flight?

And to enter the maneuver is it correct to say that the pilot gets a lot of initial forward velocity and stable forward trajectory and pulls back on the stick rather fast and holds it there till pitch authority is lost post-stall? And then just waits it out till the plane completes the cobra and returns to controlled flight in the forward direction? Or is there more to it?

@curious_cat: Yes. There might be more with specific elevator angles post-stall, but in essence that is it.

Does the elevator retain authority post-stall?

Also, how come there isn't a lot of altitude loss during the Cobra? Aren't we converting a streamlined, low drag attitude into essentially a large air brake + spoiler?

@curious_cat Your high drag results in a speed loss primarily.

@curious_cat Don't forget the thrust vector. At the top of the maneuver you're getting esdentially no lift. The engines are maintaining altitude. And I think the loss of airspeed is the whole purpose of the maneuver (besides looking cool). Turns a pursuer into the pursuee

@curious_cat: Elevator post-stall authority is very small. It will produce a drag force that is proportional to its projected area in streamwise direction. Say it is flying at 80° AoA. Rotating the fully-flying tail by -20° to 60° will change the projected area by (sin80°-sin60°)/sin80° = 12%. 12% less drag at the tail will add a small pitch-up increment.

@TomMcW Do the engines have enough thrust to maintain altitude directly against gravity? e.g.  On a Sukhoi Su-30 the powerplant is 2 × AL-31FL offering max dry thrust of 75 kN each. If I'm crunching my numbers right this will only support 15 tons. But the Empty weight of the Sukhoi is 18 tons and with 50% fuel somewhere around 25 tons. What gives? Or will afterburners be used typically during a Pugachev's Cobra?

@curious_cat After watching videos of several ac performing the cobra (like this one from YouTube) it appears they gain some upward momentum with the first part of the pitch-up. They don't appear to lose any at all during. The momentum of the climb will help. Then the power of the engines holds them there. . .

. . . I don't know how to tell if the afterburners are engaged or not but that would prob be a good guess. A few of them exhibit a trail when they are doing the cobra but I can't say if it's exhaust or vapor. I'm thinking afterburners may create a smoke trail because they might not have quite enough oxygen for complete combustion and could produce visible smoke. But that's just conjecture. Peter could prob confirm if that is true or not

@TomMcW You are right. Even the set of images illustrating the maneuver I borrowed from Wikipedia shows a slight altitude increase and then decrease. So that must be the contribution from the initial upwards momentum and the pitch up contribution. So I guess it isn't a constant altitude manouver in a strict sense. Altitude is first gained and then lost.

Continuing from this answer, the aircraft actually requires a strong pitching moment to maximize the shaded area of $C_m$ > 0 (nose-up moment) by  full stabilator trailing edge up (TEU), as shown in the Su-27 Cm curve, in order to maximize pitch rate to get into the $C_m$ < 0 (nose-down moment) area as far as possible. The $C_m$ < 0 area above 70 deg AOA, where significant nose-down moment is mostly generated by the aircraft itself, allows pitch motion to be stopped and the aircraft to be recovered from the high AOA region naturally.

Source: High Maneuverability Theory and Practice, TsAGI. (highly recommended)
So Why the area of $C_m$ > 0 matters? Because pitch acceleration is calculated as :

Where the component of pitch acceleration due to aerodynamic moments related to $C_m$, as indicated in the equation is:

total $C_m$ * dynamic pressure * reference wing area * mean air chord / moment of inertia about the body Y axes $I_Y$.

Total $C_m$ also includes other moments, such as the pitch dampening moment $C_m$q, but I'll keep the complexity to a minimum extent.
Pitch rate (q) is simply the integration of q-dot. So a larger $C_m$ > 0 area would provide a greater pitch rate, if all the other variables are fixed. This would require the stabilator to be deflected and held to the max position as soon as possible, so the Direct Link control mode is used in the Su-27. Center of gravity is preferably at its aft limit to increase the area of $C_m$ > 0.
There's not much the stabilator can do once enough pitch rate is generated above the AOA limit, say, 24-26 deg AOA, where the maximum nose-down moment available (with full stab trailing edge down / TED) for stopping the pitch motion plummets, indicating a stalled stab.
The problem with the F-16 during a recovery is probably the deep stall around 60 deg AOA, as the full stab TED is not able to generate even a negative $C_m$, but rather, the aircraft would pitch-up. Pitch rate would converge and AOA can be stabilized at the deep stall trim point where $C_m$=0, in the positive static margin area of AOA > 50:

Source: NASA TP-1538. (also highly recommended)
But this would usually happen with a relatively low initial pitch rate. When recovering from a cobra maneuver, the negative pitch rate reaches its maximum at 60 deg AOA ($C_m$=0) with either a neutral or TED stab, and can easily overcome the pitch-up region. After AOA is reduced to 40 deg, where both static margin and $C_m$ are close to zero with a neutral stab, a pull on the stick may be required to stop the pitch down motion completely.
This all assumes that the FLCS is non-existent and we have direct control of the stabilator.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Was the Space Shuttle landing sequence executed manually or automatically?
I was motivated by another SE Aviation Question where the complex set of manouveres needed to land the space shuttle is described to ask the following question:
Is the landing handled manually or is there an auto landing system? I suppose there is a high fidelity simulator to train all this on?
I wonder if that simulation engine has gotten into any of the enthusiast flight simulator software?
How does the space shuttle slow down on the re-entry, descent, and landing?

Obligatory link: https://www.youtube.com/watch?v=PdX393WawgE

Since that's really three questions, I'll address them separately.
1. Did the shuttle have or use autoland?
The space shuttle did have autoland capability, in theory. When the shuttle was being developed, it was imagined that flights could last so long that human pilots might get rusty, or have their ability to function under gravity impaired due to muscle atrophy from extended time in micro gravity.
Unfortunately, testing autoland on the space shuttle was a bit risky. If anything went wrong, the shuttle has no ability to disengage and go-around, and it might be too late for a human pilot to intervene and still make a safe landing.
They did try evaluating an incomplete implementation of the autoland on STS-3, where the lineup, final approach (outer glide slope), and preflare maneuvers would be performed by the autopilot, at which time the commander would take control and complete the landing.
The result was one of the worst landings in the history of the program. The autopilot was unable to maintain a stabilized airspeed, at first closing the speedbrake completely, then opening it again until airspeed was low, then closing again at 4000 feet which caused the orbiter to be too fast by the time commander Jack Lousma took over. He didn't really have enough time to get a feel for the aircraft or make corrections, so it hit hard and fast.
Afterwards, it was decided to discontinue testing of the autoland system, although the implementation was eventually completed.

Development of an automatic approach and landing system was terminated indefinitely due to the inability to implement an FAA [Federal Aviation Administration]-like powered aircraft certification program for an unpowered glider like the Space Shuttle. That is, an automatic system cannot be reasonably certified for an aircraft without a go-around capability in the event of an autoland system failure, because the only recourse in the Shuttle is to land rather than to execute a wave-off maneuver as in a powered aircraft. The practice of having to salvage a good landing out of a poor approach is not professionally accepted airmanship.
Further, the STS-3 experience was classified as a “late-takeover” action in which a failure of the autoland system in even closer proximity to touchdown could result in an upset too late in the landing phase to be reasonably recoverable. "Monitoring" the approach versus actually "controlling" it induces a time lapse in the mental-to-physical control conversion process which increases in criticality for manual takeover as the aircraft approaches touchdown. An automatic failure on the IGS, with no recourse for a wave-off, would be unsafe compared to a totally manual- controlled approach.

The quote is from NASA Johnson Space Center Oral History Project where Jack Lousma gives a great description of the flight and landing a little less than half way down the page.
The Columbia Accident Investigation Board Report did recommend testing the autoland system, but to my knowledge, it never was.
Just to clarify, the autopilot was used for most of reentry on every flight, but I believe your question was specifically about the final stages of flight and landing.
2. What sort of simulator was used for training?
So the shuttle was hand-flown for landing (a fly-by-wire system, so it's arguable that there's no true manual mode), and that means pilots have to be trained.
They were actually trained in several different simulators. There was SAIL (Shuttle Avionics Integration Lab) where they did serious integrated systems and software testing.
There were also fixed and motion-based simulators in Houston. These were collectively referred to as the SMS (Shuttle Mission Simulator). These were used for training for all phases of shuttle flights, not just landing. They were particularly useful for simulating contingencies, like an RTLS abort (return to launch site). It's similar to how simulators are used in the aviation industry to train for scenarios which would be too dangerous to train for in a real aircraft.

The motion-base SMS. Image Source
For nominal conditions, the best simulator for landing the space shuttle was actually another aircraft referred to as the STA (shuttle training aircraft). This was a modified Gulfstream II which would fly with its main landing gear lowered and thrust-reversers engaged to simulate the high drag profile of the orbiter. The left side of the STA cockpit was built to mimic the layout of the shuttle, including the heads up display.

Images from the STA's Wikipedia page
The STA was used for both training shuttle pilots and for evaluating weather conditions at landing sites on launch and landing days.
Lastly, there was also an on-orbit simulator that the pilots would use for practicing in the day or two before landing. It was basically a laptop with special flight-simulator software.
3. Is the simulator available in any flight simulators?
The answer depends on exactly what it is that you're asking. The actual software used on shuttle computers (written in HAL/S) is ITAR restricted, so it is not available to the public. Sadly, even the on-orbit simulator has never been released, to my knowledge.
However, there are several simulators which let you fly some or all of reentry and landing to varying degrees of realism.

SSMS 2007 is a pretty high-fidelity simulator, but has a horrible, not even remotely accurate, reentry and landing.
On phones and tablets, I love fsim Space Shuttle. It's decently realistic, but only lets you fly the subsonic portion of flight.
Orbiter, which isn't a shuttle-specific simulator, but there are shuttle plugins. I've flown several manual reentries in Orbiter, but you really have to know what you're doing or you won't make the runway. That's probably the best option if you're really interested in orbital mechanics, and a terrible option if you're not.
X-Plane has a Space Shuttle scenario, but it's not particularly realistic.
I know some people fly Shuttle missions on Kerbal Space Program, but I've never played with it at all so I can speak to it one way or the other.

Interestingly, the motion-based simulator from the SMS was donated to the Aerospace engineering department of Texas A&M, and it was discussed making the simulator at least partially available to the public. Unfortunately, I think they ran out of money to actually put the thing together and run it, so it's effectively just sitting in boxes in a room somewhere. They have a facebook page, but it's been a long time since I've seen any progress.

It's worth noting that Buran, Soviet space shuttle, did make fully automated unmanned orbital flight.

On the subject of simulators: If I remember correctly, the Astronaut Hall of Fame in Orlando had a "simulator" available for the public to try to land the shuttle. I'm not sure how real it actually is or the technology/software used. It was in a cabinet similar to a car racing arcade game that you sit in, with a joystick and screen. No cost (except admission). However, according to the site, the Hall of Fame closed in November 2015. No idea of the future plans.

@Steve There was, at least a few years ago, a (presumably) similar simulator at the Johnson Space Center.

Kerbal Space is a great game, but the physics/mechanics are not earth-like particularly.

I'd agree with @Jamiec - KSP is an excellent game but isn't "Earth-like" enough, nor are the Aerodynamics up to the level of truly simulating a shuttle landing. You can do something a lot like a shuttle landing, and it's a lot of fun, but it's not a simulator of the STS landing on Earth

Both manual and computer control were possible in all phases, but re-entry was done by computer (with manual control available as a contingency, and tested once on STS-2), while approach was almost always done manually.  No landings were ever done by computer.  STS-3 used the autopilot (with some issues) on descent.
STS-53 was to have tested autoland, but the test was cancelled.
After Columbia, further modifications were made so that with the installation of a cable to connect certain systems, the shuttle could execute re-entry, descent, and landing without pilots.  This enabled remote control of operations that would otherwise require switches to be thrown on the flight deck.

After the reentry burn, most of the atmospheric reentry and descent was handled automatically. (Exception: STS-2 was flown manually for much of the descent to collect additional data on the re-entry behaviour in various conditions.)
As for the landing, shuttle commanders were encouraged to take over at the end, after the speed had dropped below Mach 1, to get the "feel" and land manually.
I have heard that the entire landing could be performed automatically, except for the lowering of the landing gear.  For some reason this has always been under crew control.  (I can't find a reference for this though.)
For simulators, I assume a ground simulator was available.  However crew also practiced on modified Gulf Stream aircraft that could be set up to fly with a similar approach to the real shuttle.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why are planes using rivets & not welded construction?
I was motivated to ask by this question: Why are airplanes riveted and not screwed?
Why not welded construction? Is welding these alloys too difficult? I come from a refining background & in a lot of hazardous duties welded construction has become the norm because integrity is easier to guarantee than screws, bolts or rivets.
Just wondering what's different in aviation? Isn't repair, safety, weight & aerodynamics all better with a welded construction?

Short answer; It is exceptionally difficult to heat-treat a welded aluminum structure back to original strength.  In addition to all the other reasons listed below.

Short answer: High-strength aluminium alloys are tricky to weld correctly. Aluminium is such a fine material for aircraft structures that the need to rivet it is gladly accepted.
Two things are important:

While steel has a temperature range in which it gets more and more runny, aluminium alloys change from solid to liquid within a few degrees. Also, heat conductivity in iron-based alloys is lower than in aluminium, so heating steel locally will keep the surrounding material cooler and more solid compared to aluminium. While welding thin sheets of steel is trivial, it needs lots of experience in aluminium. For very thin sheets, special equipment like a water-cooled copper backing on which the aluminium sheets rest, so their back is cooled, are needed. Also, the melting temperature of steel and titanium is high enough for it to glow long before it melts, while aluminium will melt without giving you any optical hint of its temperature.

High-strength aluminium is produced by progressively aging and precipitation hardening the material. The usual alloys use copper atoms dispersed through the aluminium matrix which locally distort the atomic lattice and strengthen it. If they are heated and rapidly cooled by welding, the copper distribution would be changed and the material would be weakened around the welding area. To harden the finished structure again is rather impractical in most cases, so riveting is the better alternative.

A third speciality is the oxide layer on aluminium, which has a higher melting temperature than the base material. You need an AC TIG welder to disrupt the aluminium oxide layer, so your choice of welding techniques is rather limited.
Also, riveted structures are easier to inspect and to repair. Most repairs need to remove aircraft structure for access, and a riveted structure is easier to disassemble and to put together again after the repair using slightly thicker rivets.
My experience with aluminium welding stopped at 4mm thick sheets; while thicker ones were easy to weld, I never managed to weld thinner ones. You sit in front of your structure and heat the spot where you want to start welding. Watching it through the darkened head screen you wait until the spot under the arc becomes glossy, which signals that the surface has started to melt. Now you need to add your welding wire like crazy to keep the spot from heating more, and get the spot moving. If you fail to do so, a second later you will have a hole under your arc, because the aluminium has molten completely and has fallen away. Doing this with 2mm sheets was a pure exercise in futility for me - the moment the surface became glossy it fell away already.

Edit:
Thanks to @voretaq7 for sharing the link about friction stir welding in the comments! This is made possible by precise positioning of the parts and a computer-controlled welding head and will see wider application in the future. Eclipse Aerospace claims it helps them to avoid 60% of rivets in their jet aircraft.

Years ago I had an engineering materials professor, specializing in metals and allloys, who, in place of the term "welding" for the chapter, used the phrase "The Ruination of Otherwise Carefully Prepared Metals".

Thanks Peter! In aircraft construction what are the typical sheet thicknesses one encounters? I suppose fusalage  vs wings vs control surfaces all might have very different thicknesses and depending on Type too but I was just wondering about some general values. If welded wings / fuselage etc. was to happen are we looking at 2 mm or 4 mm or much thicker sections?

Also, this sounds like an application where robotics might help? Has it in reality? i.e. Getting the exact temperature and then rapidly and precisely placing the weld bead where it needs to be. How much of welding in aircraft construction has been automated? And has that helped any in making welded Aluminium more practical?

@curious_cat: Computer-controlled welding is certainly a big step forward - mind you, compared to the TIG welders 30 years ago the new ones with automatic voltage adjustment for starting the arc and current control are a revelation. And that is still manual welding. But for the computer to do the job correctly, everything has to be carefully prepared. Regarding thicknesses: Most aluminium is less than 2mm thick. It is more efficient to use a paper-thin sheet and rivet reinforcements to it than to use a thicker sheet.

Thanks Peter. By reinforcements you mean ribs, angles etc? i.e. Some form of stiffener? If so, is buckling the dominant failure mode that's controlling for say fuselage? Because, I'm thinking if internal pressure is the dominant mode than the controlling stress (hoop stress etc.) ought to be dictated by diameter & sheet thickness. In which case stiffening shouldn't help, correct?

@curious_cat: Yes, stringers, ribs, stiffeners, all what needs to be done to stiffen sheet metal. Of course, if the stresses demand, you will need thicker gauges. Internal pressure is just one case; dynamic pressure in supersonic structures is another. Most GA aircraft skin thicknesses, however, are sized with ground handling in mind: Pilots and mechanics must be able to push the aircraft without poking holes into the skin with their bare hands.

Thanks Peter. Amusing how the most mundane use case if often the governing one for design.

It's worth nothing that some aircraft are constructed by welding - at least in part. The increasing use of composites seems to make this less likely though.

I have observed in the A320s the nose area is welded, is it made of steel?

@Firee: I do not know the details of the A320 construction. The nose houses the antenna for the weather radar and should thus be a composite part. Composites are joined by bonding, and joined to a metal structure by bolts. But maybe there is some welded support structure.

@PeterKämpf: I am no expert in welding too.. It just looks likes its welded, could be some kind of bonding too..Has a thick line at the joints..Definitely not riveted

@Firee: If they use flush rivets, you need to be really close to see them, especially after the part has been painted. The  thick line could be a sealant - most rivets today are set "wet", with a sealant between the parts. This sealant will also transmit some shear and provide a corrosion protection. If the sealant has been applied generously, it will look very much like a welding seam.

Laser beam welding has already been in production for some time for joining panels,  e.g. at Airbus. This is used on compatible Al alloys. While standard 2024 duraluminium does not weld well, these do. New alloys like AlMgSc are especially suitable for welding, as they don't need a heat treatment to avoid recrystallization. In addition FSW introduces much lesss heat and avoids melting and all it's drawbacks completely.

peter, this was a most informative and complete discussion!

Welding has 2 significant drawbacks (1) it generally weakens the parent material the heat in affected zone and (2) use of a single component can lose crack stopping at the change from one section to the next. I heard a story that someone (Lockheed?) had tried the use of diffusion bonding but then suffered major crack development during structural testing.
Having had access to the Comet fuselage used for water tank pressure testing (mounted on the wall of a lab at RAF College Cranwell) the growth of cracks is clearly apparent. In the past, engineers used to drill holes at the end of a crack (when they found them) because the increased tip radius slows the rate of propagation. My father and godfather were both Imperial Airways engineers in the 1930's, 40's and 50's. (OK, the name changed to BOAC i 1940.)
Also, it is almost impossible to test the integrity of a weld without breaking it; like the match factory that tests 100% and only despatches the ones that worked.

Re. Weld Integrity: What about X-Ray Testing, ultrasonic testing, Radioactive Testing etc.? Are they not trustworthy enough for weld integrity tests?

I saw a recent documentary on the Comet, where they went back and re-examined the aerial cutout section from G-ALYP that was recovered, where the crack that destroyed the aircraft was believed to have started. What they found is that in addition to the almost square openings, a wartime riveting technique had been used, that created micro cracks in the skin around the rivet holes. That wasn't a problem for the unpressurized bombers made for the RAF, but was a problem for a pressurized airliner.

@tj1000: The punch-riveted fuselage wasn't just a major contributor to the disintegrating Comets, it was pretty much the only significant contributor.  If you take a look at the Comet 1's windows, you'll see that they were already quite rounded - certainly not significantly more angular than those of many other jetliners (look at the Caravelle's triangular windows, or the rounded squares or rectangles used nowadays). (1/2)

(2/2) Besides, the Comet's overhead ADF windows (the original suspect in the breakup of both BOAC 781 and SAA 201) were of identical shape and construction (apart from being filled with opaque fiberglass instead of transparent panes) to the cabin windows, and remained rounded squares, even on the later Comets with circular cabin windows... and, yet, no later Comet came apart in midair as a result of fatigue cracking emanating from the ADF windows.  It wasn't the switch to circular windows that saved the Comet 4 - it was the elimination of punch riveting.

Actually, it was a combination of both the poor riveting technique starting the cracks, and the squared holes accelerating the stress. In the original tank test, the Comet fuselage split open around a window opening, while Yoke Peter appears to have come apart around the ADF cutout. They would have failed due to the riveting cracks alone, the somewhat squared openings accelerated the process by increasing the stress. Conversely, had the cracks not been present, the squared openings wouldn't  have been as much of a problem.

I came to this thread in search of comparisons between rivets and welds. My curiosity was piqued in reading "The night the Fitz went down" / by Hugh E. Bishop in cooperation with Dudley Paquette. Paquette (captain of the  Wilfred Sykes when the Edmund Fitzgerald went down) was critical of welds in ships, which have become almost standard since a comparison of  of welds vs. rivets done when the Navy was trying to decide how to build submarines. The Navy concluded welds were much stronger than rivets, so they started welding subs and m\pretty much all the big boats after that. Paquette didn't trust them, because a big boat "works" (flexes and twists) a lot, particularly in storms, and welds are so rigid that where a rivited area would flex with the rest of the ship, a weld would pop right open.
I'm not entirely sure how applicable it is to planes, but it goes to show that "boats and ships do it" may not be as strong an argument as people may think.

I work in the shipbuilding industry, I was onboard the USCGC WLBB-30 Mackinaw when it struck a seawall (I actually have a piece of the dented steel they cut out and gave to me) on it's maiden voyage. The welds held very well. Welds themselves are actually stronger than the surrounding steel.

Yes, that's what I said that the Navy concluded. I AS

Yes, that's what I said that the Navy concluded. I ALSO said that the welds were more rigid, i.e. more prone to brittle fracture. Unless there's a major snafu in construction, any join will perform well on a maiden voyage. But consider this: there were four big boats on the lake on Nov. 10, 1975. The Wilfred Sykes had just been converted to a self-unloader, the Arthur M. Anderson had just been lengthened, and the Roger Blough was only three years old. Guess who didn't have new steel in her (and a loose keel, to boot)?

First of all, most aircraft have many welded components, but they are mostly the frame/structural components. The skin is usually rivetted for two reasons (1) it is much easier to remove or replace a rivetted panel, and (2) the skin-to-structure mating location is in many cases not accessible. In other words, the skin needs to be joined to the frame. That means the welder would have to be able to access that joint. Even if you had access to the joint, it would just be a seam weld, then you would have to weld again on the outer surface to seal the butt joints between panels. Then you would have to grind those seals. Very time consuming and expensive and probably not stronger than rivets anyway.
Also, consider that airplanes flex constantly while they fly. This will play havoc with welded joints, potentially leading to cracking in a thin joint (which skin joints will be). Rivets are much more friendly to moving parts because they have a little give to them.
Normally, you want a lot of surface area for the weld, and with a thin section it is hard to get this. For this reason it would actually be a much better idea to solder or braze the skin on than weld it.
All those things being said, it is very possible to design a plane using welded construction similar to a car using what are called "spot welds". This may yet happen, but the aircraft industry has not got there yet.

Spot welding is not too popular/useful as singular method for lightweight construction, as it introduces load peaks. Even today in the automotive industry, spot welding is mainly used together with adhesive bonding to quickly fix parts in place until the adhesive cures to provide structural strength. Line welding is quite well introduced already (see above,  Laser beam welding and increasingly friction stir welding).

Just wondering what's different in aviation? Isn't repair, safety, weight & aerodynamics all better with a welded construction?

I just thought I'd extend with another thing that is used extensively (which you may or not know about since it is less obvious): Adhesives.
While not combining the material in itself like welding, it goes along the same lines but unlike welding generally does not change material properties. Industrial strength adhesives, such as epoxy, can be incredibly strong when applied properly.
This PDF from Henkel has a huge range of products as does this overview from 3M Aerospace.

Among structural engineers there is a saying: Those who don't know rivet, those who know already something use adhesive, and those who know a lot rivet again. Modern rivets are set wet, meaning that at least some sealant is applied. Ideally, this sealant sticks well and transmits shear. Now you have a bonded joint secured with rivets. That is really the best way currently. Bonded joints must be secured against peeling by adding rivets.

@PeterKämpf Maybe that's one advantage of welded over adhesives. Welds don't peel. Are there peel resistant adhesives that need no riveting?

@curious_cat: No, all there is are peel-resistant design techniques. But peel forces are impossible to avoid, especially when damage starts somewhere and begins to spread. Rivets help to stop the progression of damage. You can lower peel stresses somewhat by spreading them out by using a thick layer of elastic adhesive, but the strength of this kind of adhesive is rather low. The other way is to stiffen up the parts at the ends of the bonded joint.

In refinery industry ; welding is not easy, and that is for steel which is much easier to weld than aluminum and titanium. ASME  Boiler and Pressure Vessel code has a whole section ( Section 5 ) that defines the requirements and conditions for welding refinery steels. One of the biggest problems for any metal is the heat affected zone ( HAZ). The metal in the HAZ has seen the whole range of temperatures from melting to nothing. For aluminum, the weld deposit and some of the HAZ is in the lowest strength condition . I will be brief. To bring the aluminum back to strength , the entire welded structure would need to be solution annealed ( heated to about 900 F ) and rapidly cooled ( like in water). Then aged at about 300 F. This is the common  T 6 condition . Consider the problems of supporting the whole structure while hot and the aluminum has the strength of cheese. And then quenching in water. Titanium has its own set of unique welding problems.

As others have noted, it's because of the properties of aluminum. I'd like to add that there are good alternatives to using aluminum but the only one that's feasible on both performance and price is magnesium.
Northrop's rocket powered XP-79 used a welded magnesium monocoque structure for example.

Depends how you define "performance." It reacts with water and is highly flammable, both qualities that are fairly sub-optimal in aircraft structures.

@fooot A magnesium alloy called Elektron was very popular in German and Russian aircraft. Corrosion was the biggest problem, and flammability was no issue.

@fooot "sub-optimal" heh... :)

@fooot Mg is used for helicopter gears and components. Whlie corrosion can be problematic,  burning is not. In terms of ductility and formability,  Mg is no match for Al. Especially forming thin sheets is problematic.

authors: Дмитро Дмитров


text:
What, effectively, does a navaid being unmonitored mean?
I saw a NOTAM about a VOR being unmonitored. I think this means that the voice capability of the VOR frequency is not in operation/nobody is listening. Is that right? If so, it just seems like something nice to know but doesn't affect the navigational capability of the VOR, right? If a VOR is unmonitored can I still use it for navigation, such as for a VOR/DME approach?

When I took my initial instrument checkride, I was flying the VOR-A for a rural airport, bouncing along, and working at "ATPing" it by keeping the needles inside the donuts.  The VOR had been identified, and I was turning final, if one call it that, for the VOR-A.  Then I noticed that the ID for the VOR changed, and was "TST" which I recognized readily as I know Morse code.  I applied full power and executed the missed, announcing what I was doing on the way.  The cranky DPE wanted to know why I did the missed, and I told him the VOR was unreliable.  I managed to pass the ride.

Actually it doesn't have anything to do with voice capabilities:

5-3-5. UNMONITORED NAVAIDs
a. All VOR, VORTAC, and ILS equipment in the NAS have automatic monitoring and shutdown features in the event of malfunction. Unmonitored, as used in this order, means that the personnel responsible for monitoring the facility have lost aural and visual monitoring capabilities and cannot observe the status of the facility. It does not refer to the automatic monitoring feature.
b. When a navigational aid's operational status cannot be monitored at the controlling or monitoring facility, but all indications or reports are the facility is operating normally, issue a NOTAM placing the aid in an unmonitored status.
c. When issuing a NOTAM describing a facility as unmonitored, do not use the category of monitor, only the contraction UNMON.
EXAMPLE-
!DCA LDN NAV VOR UNMON

(Source: JO 7930.2M)
So basically it says that this VOR may malfunction without notifying a responsible party, or that a responsible party is no longer monitoring the VOR. A lot of times this is done with VOR's that are scheduled to be removed from service, they may be allowed to continue to operate but are no longer maintained.
You can still use the VOR for navigational purposes. The VOR is supposed to shut down in the event that it becomes unusable, so you can be (relatively) assured that if the VOR is operating, it is reliable and usable.

Of course, if the airport authorities aren't doing their job properly, the automatic monitoring feature can also fail.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Why are piston engines for aircraft rarely diesel powered?
Why hasn't diesel caught on much for aircraft engines?  The list of diesel aircraft engines on Wikipedia is very short.
The reason I see mentioned is their poorer power-to-weight ratios. (e.g. What are the advantages/disadvantages of diesel/Jet A-1 piston engine?)
I knew this was true in the past but is it true even today? Does anyone have values comparing the power-to-weight rations of typical diesel vs gasoline engines, say for automotive use?
Also, doesn't the better efficiency, especially at higher altitudes compensate for the poorer power-to-weight ratio by allowing lesser fuel and more payload?
50 years ago diesel engines for automobiles were uncommon too but in the last decade they have been steadily making headway in the car market. Do the same developments translate to the aircraft engine  selection too?
Other than weight, are there any other disadvantages of diesel?

Actually the diesel engine is on its way out in Europe:
https://www.theguardian.com/environment/2018/feb/27/german-court-rules-cities-can-ban-diesel-cars-to-tackle-pollution

@Brilsmurfffje That doesn't compare to the leaded gasoline piston aircraft use. Not only is banning leaded avgas a priority for some governments (US EPA), the single company who still makes tetraethyl lead has talked about ending production. It is also impossible to find avgas in some places like Africa because of no demand, and the fact that leaded fuel has to be kept completely segregated from auto fuels. The point of an aircraft diesel is to burn Jet A.

You are right, there are few modern aerodiesels around - so few in fact that we can go through the brief list one by one:

Thielert: This is a development based on Mercedes-Benz car diesels, of which more than 1500 1.7 liter engines were built. Currently, a 2.0 liter engine is offered. The original Thielert company went bankrupt in 2008 and has been taken over by Continental.
Austro-Engine, another conversion of a car diesel for aircraft use. It is the result of the disappointment of Thielert's main customer, Diamond Aircraft, with the record of Thielert's initial Centurion 1.7 engine.
SMA:, a French turbocharged diesel with 5 liters displacement. Currently, around 100 of them have been delivered after almost 20 years of development work.
Zoche: The product of a father-son team with many clever details, but a quarter century of development work has not yet resulted in an airworthiness certificate for their two-stroke radial diesels. While Zoche claims to be the victim of a conspiracy by the authorities, the accused complain that the engines could not keep their cylinder heads on for long enough.

What has held the diesels back for so long?

Cost: Development in aviation piston engines has effectively stopped with the jet age. The existing designs were simply good enough until fuel prices went through the roof. This made diesels only attractive in niches, and as a consequence, production runs were far too small to justify the major development effort. Aviation engines run constantly at around 70% of their full power, so regular car engines could not be used without heavy modifications. As a consequence, a conversion kit can cost you $89,000!
Weight and complexity: The higher compression ratio of 1:30 requires a heavier structure, and fuel injection, a gearbox and turbocharging push the part count well above that of a gasoline engine. Once you compare system mass (fuel plus engine), the diesel wins only at ranges which are now mostly flown by jets. Yes, fuel flow is less (10 gallons per hour for a diesel against 13.5 gallons per hour for a comparable gasoline engine), but will only help at ranges above a couple of hundred miles.
Vibration! With that compression ratio, every ignition causes a three or four times heavier jolt to the engine and the aircraft structure. Not only the engine, but the whole aircraft structure needs to be designed with that in mind.

To be fair, a modern gasoline engine will also benefit from fuel injection, a gearbox and turbocharging, and Zoche initially did not use an electric system, relying instead on pressurized air for starting the engine (a pressure bottle loses much less of its stored energy when cooled to -20° than a battery, after all), but the practical aerodiesels are a lot more complex than the engines they are meant to replace, creating an obvious reliability disadvantage.

Thielert Centurion 1.7 in a Diamond Twin Star (picture source)

accused complain that the engines could not keep their cylinder heads on for long enough ...awkward.

@egid: Zoche had problems with cylinder heads popping off the engine in test runs maybe 20 years ago. This had been used against them years later. Generally, certification regulations are now so demanding and complex that any new entrant feels stonewalled by the bureaucracy.

There are other maintenance problems as well, such as high failure rates of fuel rail servo motors (I'm not naming which manufacturer). But, for an operation that is faced with the prospect of AvGas that is 350% the price of Jet-A, the extra cost and hassle can be worthwhile, regardless of range.

@PeterKämpf Can we state that petrol engines are generaly more relible then diesel engines, and that is in aviation demand number 1.?

@user207141 Not when compared on an equal basis. The Jumo 205 was very reliable, and diesels are very robust and, lacking fancy stuf flike carburators and electric ignition, also very hard to kill. Just consider what you need to do in order to stop a diesel from running!

@PeterKämpf Problem is today modern diesels have all this shits; DPF, EGR,injectors, high pressure pump(2800 bar!) that fail all the time.. I see aviation petrol engines have high displacment, does aviation use small displacment petrol turbo engines,like modern cars(2.0 TFSI) ?

@user207141 They tried and failed (PFM3200). Please note the "equal basis" condition. Modern gasoline car engines also have all those gizmos.

Although this has partially been covered in this question, it's different enough to warrant an answer.
One word: cost
In the vast majority of light piston aircraft the benefits of diesel are way lower than the costs in involved.
Diesel engines are heavy, this is because the diesel cycle relies on cylinder pressure rather than a spark to ignite the fuel-air mix. This means that diesel engine blocks have to be at least part steel, as aluminum is not strong enough to contain the pressures. All the aero-diesel engines I have read about have a hybrid steel-aluminum block, which is harder and more expensive to manufacture. The weight of the engine has the following impact on costs:

You have to have a variable pitch prop in order to get the same amount of performance
The extra weight up front means that you have to re-arrange the balance of the airplane, moving parts from the front to the aft
Diesel fuel requires a complete re-working of the fuel system, so the wings have to be completely opened up for new tanks and the new piping. Some of this piping is to cycle warmed fuel through the tanks to prevent the fuel from gelling at low temperatures
Although diesels do not require a spark they are heavily computerized, so they require steady electrical power to operate. 2 batteries are required for redundancy. More weight, more cost
The aircraft has to have a STC (supplemental type conversion) in order to be legal. If your airplane doesn't have one you'd have to pay for it yourself, which costs tens of thousands to millions of dollars to do
Your dashboard has to be modified for the new instruments and controls

So not only are the engines more expensive in the first place, fitting them is very expensive as well. Unless your airplane is going to fly a lot it would take decades to recoup the costs. Many of the other downsides (lack of Jet A1 fuel at many GA airfields, lack of engineering support for the engines, etc) would go away if it was cheaper to get a diesel installed.

A diesel engine does not need variable pitch propeller any more than a gasoline engine does. Only turbines require variable pitch propellers. Diesels have lower optimal RPM, but those built for retro-fitting come with gearbox to provide the same propeller RPM as the original engine.

Diesels don't have magnetos, because they don't need electricity. If they are computerized, it is because they are new designs, not because they are diesels—gasoline engines have more use for a FADEC than diesels.

Some of your answer is based on modifying an existing aircraft to fit a detail. I think the op is asking more about designing ac for a diesel engine in the first place. It does sound like most of it still holds true, though

@GdD -- aerodiesels use Jet-A, so gelling isn't an issue :)

See British Airways flight 38 @UnrecognizedFallingObject what happens when Jet fuel gets too cold.

Aerodiesel engines do need electricity @JanHudec, just not for the spark. While some kit aerodiesels have a "limp mode" and still run without electricity all of the certified ones are electronically fuel-injected and computer controlled. They simply won't operate without electricity.

@GdD if by certified you exclude supplemental certification, you might be correct. However the SMA diesel does have the manual emergency mode that does not require electrical power.

@GdD -- re: BAW38 -- you generally don't get OATs that cold at GA-prop altitudes unless you're in arctic areas where you'll be buying Jet-B anyway.

not to mention cost of FAA certification...

The issue with weight to HP has been surpassed by many manufacturers already. There are 200hp diesel engines in the market, that are smaller and weigh less than its counterparts from Lycoming or continental. Actually, their frontal dimension is smaller than avgas engines, providing for the possibility of smaller and more aerodynamic cowlings resulting in less weight and higher performance. This engine can further be enhanced by using MT propellers that are lighter than traditional propellers and provide better climbing performance.
The cost of the Diesel engine will go down as did computers with more volume. If we consider that jet fuel can be 50% to 60% cheaper and some engines are 30% to 40% more efficient than avgas engines, it merits for new airplanes to consider the use of diesel and abandon the close to the 100-year obsolete technology of the avgas engines.
Furthermore, diesel engines lend them self for future use of alternate fuels and biodiesel that could be produced cheaper than biofuel for avgas-burning engines.
Until the electric technology for aviation does not reach a point of balance between speed and range, I consider that the future for us real pilots that have to cope with ever increasing costs of flying would benefit from lower operating costs if Diesel airplane would be available.

Welcome to aviation.SE. As it is currently written, this seems more of a wish list about what diesel engines might become. could you rephrase it a little to address more explicitly the main question?

"The cost of the Diesel engine will go down as did computers with more volume" - it's more complicated than that. Cost does go down with production volume in general, but the major factor in lowering the cost of computers was Moore's law which applies only to computers.


text:
What happens to Flight Testing Aircraft after Testing is over?
Are aircraft used for flight testing typically sold to Airline customers after their use for flight testing is over?
e.g. The aircraft shown in this SE Aviation Question:
Why are these windows on some airplanes red?
Sounds like a lot of money to waste so I expect they would be sold for some revenue but to what end use is the question.

Note that there are also airframes (mostly the fuselage of the aircraft, and some of the systems, but not all, so they're not full aircraft) which are actually actively destroyed during testing.

It depends. In most of the cases, the aircraft have been instrumented and pushed so much that it not possible to modify them for commercial flight. In these cases, they are either preserved or continued to be used as test aircraft. In other cases, the aircraft are modified and delivered.

Boeing 747- The first ever Boeing 747 was used exclusively as a test aircraft and was later handed over to museum. On the other hand all the 747-8s were converted back after test flights and delivered to customers.
Boeing 777- The first 777s were considered production models, not prototypes and were delivered to customers after testing.
Boeing 787- It is kind of a mixed bag here- The first three 787s are in museums across the world (two in the US, one in Japan). Boeing is still using one as a testbed, while one more is in storage. Another aircraft used for testing was later modified and sold to the Mexican Air Force.
Airbus A350XWB- Right now, Airbus is holding all the five testing aircraft and using them for testing or demonstration flights.
Airbus A380- Airbus has all the test aircraft.

You can check out the production lists of various aircraft here

Boeing intended to deliver all the 787-8 test frames to customers, but in the end they proved to have deviated so far from the production certificate standard that Boeing transferred their cost to the R&D budget and wrote them off. Boeing is still trying to place all "terrible teens" that so far havent been taken up by the customers they were intended to, so it has an excess of airframes around to play with.

Prototype aircraft are generally of less value than the production counterparts, since they are heavier and may not have the normal design configuration.

Boeing 737: The original -100 ended up going to NASA as a research aircraft as NASA 515.
Boeing 747: At the Museum of Flight.
Boeing 757: Nicknamed "Catfish", it is an experimental avionics testbed registered N757A.
Airbus A320: One retained and continued to be used for the type's winglet/engine testing.

Particularly new aircraft (and derivatives thereof) tend to go to customers as testing becomes more refined and the cause of less design changes:

Boeing 737-800 prototype went to commercial service, now operating as TC-SNY.
Boeing 737-900 prototype went to Alaska Airlines as N302AS
Boeing 777-200 prototype went to Cathay Pacific as B-HNL.
Boeing 777-200LR prototype went to Pakistan International Airlines as AP-BGY

However, not every aircraft can be sold if the aircraft is too unattractive and end up in a museum so many have been scrapped.

If I am not mistaken, PIA was the first customer to receive the 777-200LR.

Like many of these answers mentioned, it depends. For instance, the 787 program did write off the airframes for the first 6 aircraft due to the extensive number of modifications that were required to those airframes. They do go to museums of strategic importance to either the manufacturer or the kick-off customer and their country. Some of these aircraft end up going into lifecycle fatigue testing or are maintained in the ownership of the manufacturer for follow-on testing that would be for upgrades or newer subsequent models. If the program is a brand new model or significant technological change to an existing model, it has to go through stability and control testing and flutter testing. That literally beats the airframe to hell. It will most likely experience an over-G loading in these tests, and therefore, cannot and should not be used to carry passengers. There would be a potential for cargo in these cases, but then there would be added maintenance costs and operating costs from being structurally modded and heavy. There aren't many that would want to purchase a 'new' aircraft that had gone through the beatings these aircraft take.

In order to reduce the certification time needed, manufacturers usually manufacture more than one testing aircraft, this way several flight tests can be done in parallel.
Once the airplane is finished, keeping those airplanes is a cost and there is no business case to keep all them. However, an airplane model is a living design and during its lifetime (could be more than 50 years, as B737 for example) the airplane design will be updated.
Things as simple as obsolescence of equipments, small improvements, new technologies... not just re-engining the airplane. Those modifications must be certified as well, and some of the means of compliance require a flight test.
So, in this context:

Manufacturer usually keeps always one of each airplane model, and likely more than one during a period of time.
Those having less damages and usable for commercial flights (like long haul testing) are usually sold with a discount (and actually very early...)
Finally, those that are too expensive to keep and not usable for commercial flight are scrapped or sold/donated to institutions.

Anyhow, is a significant amount of money what is lost, but is part of the development cost. Finally is a business case... you can extend the certification period by 1 year and save 2 testing airplanes? Having 1 year more the manufacturing line stopped waiting for the certification? Finally using several testing airplane is a possitive business case.


text:
What is the purpose of a departure procedure?
On many webpages it is said that a departure procedure will keep aircraft away from terrain and obstacles and avoid collisions. Which type of obstacles? Buildings? Towers? What is the purpose of a departure procedure? This is a follow up of this question.

Any and all obstacles. They also help ATC by providing standard routes through potentially busy airspace. Their purpose is to facilitate the safe and expeditious flow of traffic.

They also use departure procedures for noise abatement.

Another reason is so that departing traffic can gain height where there is no inbound traffic (see standard arrival route) so that when they leave the departure procedure, and cross inbound traffic, they are higher.  SIDs and STARs work together to keep traffic separated.

Most of the time, the crew will use a standard departure (SID), which is a pre-established and published route with altitudes. This decreases the number of exchanges between crew and ATC. See on Wikipedia. Departures are the link to access airways used for cruise.

A major advantage is standardization. An aircraft can be given a departure procedure, and assuming everything goes to plan, everyone involved will be on the same page as to what is going to happen. When air traffic control instructs an aircraft to fly a departure procedure, a large volume of information (e.g. climb to this altitude, turn this way, turn that way, etc...) can be communicated just by giving the name of the procedure. This is far more convenient and predictable for both the crew and controllers than giving every instruction individually. This allows controllers to focus on preventing conflicts and dealing with abnormal situations rather than issuing routine instructions, and it reduces the risk those instructions may be miscommunicated or misunderstood.
The procedure itself will have been developed by specialists based on all sorts of criteria, among them:

Obstacle clearance. This can be anything from the huge mountain range near the airport to buildings to towers to anything else. The procedure can be designed with a required minimum rate of climb, and pilots can ensure they are able to meet that requirement to climb over any obstacles. Obstacles might also be virtual, such as restricted areas for military purposes.
Noise abatement. Communities in the vicinity of airports often have noise complaints. Departure procedures can be designed to direct aircraft away from noise sensitive areas.
Traffic flow. Near a busy airport, large numbers of aircraft are maneuvering at low altitudes. Standardizing departure and arrival flows along certain corridors allows air traffic control to deal with aircraft moving in predictable ways and avoids potential conflict between different aircraft. Departure procedures help ensure separation between arriving and departing aircraft.
Regional traffic flows. In congested airspace where several busy airports are located in close proximity, procedures can be developed to ensure that traffic to and from each airport are kept separated. A good example is the busy airspace around New York City, where complicated departure and arrival procedures are used to separate traffic into the region's airports.
Winds. Different runways may be used depending on wind conditions. There may be different departure procedures available for different runway configurations and/or universal departure procedures that have different instructions depending on the runway used.
Navigational aids. While modern RNAV procedures are now available, traditional departure procedures guide aircraft from the runway through routes navigable through NAVAIDS like VORs and NDBs onto established airways.

Of course, an aircraft flying a departure procedure can always be given instructions to deviate from that procedure if it becomes necessary, such as for other traffic. If conditions permit, it's also possible for a controller to give a "shortcut" by circumventing part of the procedure, perhaps by turning early toward the next waypoint on the flight's filed route.


text:
Why don't stealth aircraft designs use podded engines?
Normally, it seems all the stealth designs dont use podded engines like others. Is this just to avoid the radar cross-section?

Fighers generally don't use podded engines. A-10 is the only combat aircraft I can think of that has podded engines and it is not a(n air superiority) fighter.

Yes. From the PoV of stealth aircraft, the podded engine has significant disadvantages:

The engine pod has a high Radar Cross Section; In fact, stealth aircraft bury the engine inside their fuselages to minimize radar cross section. For example, the B2 Spirit engines were located inside the fuselage, as shown in the schematic below.

The exposed compressor blades in the podded engine reflects radar waves, which is not good. For example, in Boeing X-32, the location of the engine meant that the compressor blades will be exposed to the Radar and baffles were (proposed to be) added to reduce the Radar cross section. On the other hand, it can be seen that it is practically impossible to see the F-22 compressor blades from any angle of the inlet.

Source: f-16.net

In case of podded engine, it is difficult to shield the exhaust thermal signature. Compare the infrared signature of the typical airliner in the following picture with that of the B-2 Spirit in the next picture.

Source: laserfocusworld.com

Source: bestfighter4canada.blogspot.in
Of course, contemporary combat aircraft don't use podded engines at all (with the exception of the A-10, which is a ground attack aircraft).

In addition to aeroalias's very detailed answer, I think a good point to start with this question is: "Why would any aircraft not use podded engines?".
There are a few advantages of having an engine in a separate pod, commonly it comes down  to a simple lack of room in your fuselage. Either your thrust requirements would require too many or too large of engines to fit streamlined in a body that fulfills your other specifications. So, we see these on things with massive thrust requirements, which are almost exclusively cargo/transport or bomber aircraft. There are a few other bonuses too, like accessibility for maintenance or removing the sound and heat from the body of the aircraft.
Unfortunately, they also present massive, super-hot, and exposed targets. So for any combat aircraft that has the option, they will be onboard. The only combat aircraft I know of with podded engines, the A-10, has to push a very heavy airframe around with high mission loads. The main body is kept remarkably thin, thanks to the lack of internal weapon bays, so it makes more sense to mount these two large engines in pods then to grossly swell the body to fit them. It also gives more freedom in the placement and alignment of these engines, which are a bit particular for the A-10 given its unique lift and weight distribution.
There are additional reasons why you would avoid mounting these on a stealth aircraft, but for airframes like the F-22 or F-35, it wouldn't have made sense anyway. Large stealth aircraft like the SR-71 and B-2 are sleek enough that the requisite engines can fit rather comfortably within the design of the wing as well, so podding them would only hamper the design.

Also, the air intake for supersonic aircraft needs to be relatively long, to reduce the airspeed to subsonic before it reaches the engine. It may contain internal moving parts, and/or use the geometry of the airframe ahead of the intake to control the position of the shock waves as the airflow slows down. A podded engine design would make those features impractically big and heavy.

Ease of maintenance would be the main reason for putting engines in pods, which is presumably why all modern jet airliners use that scheme

The A-12's engines are podded partly for field replacement.  The port and starboard engines are IDENTICAL (not symmetrical).  That is to say, one of them is mounted upside down.  As a result, they are interchangeable and can be field swapped.  They built it so that if you have two A-12s stuck on the ground each with one bad engine, no matter which engine it is you can turn those into one A-12 with two good engines.
It was designed for grimdark WW3 with Russia in Eastern Europe.  Its designers expected that it would be necessary to cobble damaged planes together at airstrips to continue the fight.

Welcome. Does it answer the question? It seems more useful as an (interesting) comment highlighting the A-12 as an exception to the rule, adding to @Jan Hudec 's one.


text:
How is the visibility at airports measured?
What tools are being used by airports to measure visibility precisely?
Do we have some kind of tools on aircraft as well for measuring visibility?

The visibility is measured alongside the runway, usually at various positions, and is called the Runway Visual Range (RVR).
RVR is defined as the range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line.
RVR is measured using so called transmissometers.

source: keesfloor.nl
These units transmit a beam of light (usually using laser) to a detector. In some case the transmitter and detector are in the same unit and a reflector is used. The ratio of the transmitted energy and the detected energy is a measure of how much light is absorbed in the fog. From this the RVR can be calculated.
Another type of sensor used to measure the visibility on airports is a forward scatter sensor. This type of sensor can be mounted on a single mast and gives more accurate results for visibilities > 2 km. This kind of sensor is used for both determining RVR and Meteorological Optical Range (MOR).

source: KNMI (PDF)
This type of sensor works by detecting how much of the transmitted light is scattered into a detector which is placed off-axis.

Aircraft don't have means to measure the visibility other than the pilot Mk 1 eyeball.

Since light reflects off the water droplets in the air, the forward scatter sensor determines visibility as the inverse of the amount of light detected at the receiver? i.e. more light received means more fog for it to have bounced off of? If not, I'm totally lost on how that would work...

@FreeMan: That's correct. In perfectly clean air, the off-axis detector would not see any light.

And the other one is, controllers in the tower using fixed distance markers, to show how far they see.

To expand a little: "Ground visibility" and "Flight visibility" are 2 distinct things.  Ground is measured as mentioned above, and the sole source of flight visibility is what the pilot reports (measured with above mentioned eyeball)...

Moreover, when ground visibility is reported (automated or observed), a pilot MUST use that. I have been grounded when there is fog over the sensors, but not on the runway.

To expand on an earlier answer at some towers Controllers use prominent landmarks in the distance to give a rough guide to RVR. They know if they can or cannot see a particular building what the RVR is. This is only used as a last resort when equipment malfunctions, but it works.

Great addition! Welcome to Aviation.SE Writerchris.


text:
Does installing a PTZ dome camera outside the aircraft violate any FAA rules?
I want to install a big PTZ camera at the exterior of the plane like A380 which can give the pilot a clear view while landing or taxing.
I have gone through a similar question but it doesn't give any specific answer though. Can I attach a camera to the outside of my aircraft in the USA?
My question is: if its going to be a permanent part of the aircraft and not a GoPro type, can it be included by the Manufacturer themselves?
I am not sure if this kind of exterior object is going to affect the aerodynamics of the plane. I got confused when an aerospace engineer told me it can affect the plane and can create some issues whilst in the air.
I would like to get clarification on this matter.

Just an aside: "I am not sure if this kind of exterior object is going to affect the aerodynamics of the plane" - if you change the shape of the aircraft (in any way) you will affect the aerodynamics...

yeah i agree, but I am not sure if the impact will  be very minimal or more in case we put something like a dome camera outside of the aircraft..

Aircraft out there already use external cameras for use during taxiing/landing. Airbus uses a system called external and taxiing aid camera system (Etacs) in A380 and A350, which uses three cameras:

One camera in nosewheel well gives a view towards the aircraft’s nose gear.
A second camera mounted on the vertical tail gives a high overview of the aircraft from the tail.
Another camera is situated on the belly of the aircraft for giving views of runway below.

Note that both of these are FAA certified and under operation.
Whether or the camera affects the aircraft performance depends on the aircraft and the camera used (and installation location), though in most cases it shouldn't. In that case, you may have to get a supplementary type certificate


text:
Can I use the private pilot written exam for sport pilot?
I'm planning to first get my sport pilot license and then continue on to private pilot, to save on written exam costs is it possible to just take the private pilot written exam for the sport pilot license or will I have to take both?

No.  You will have to take both written and practical exams for sport and private pilot, if you go that route.  I’d recommend that, if you do ultimately want to obtain a PPL, that you not pursue an SPL as part of your training and solely concentrate on obtaining your private only.  You can exercise all the privelages of an SPL with a PPL, and it cuts out the excess costs and time with the SPL testing and checkride prep.

+1 and generally I'd agree but it's worth noting that a private certificate requires a medical, whereas a sport certificate doesn't. For some people that's a really important consideration.

Indeed.  However if this applicant wants both an SPL and a PPL, he/she will need at least a third class medical certificate to take the PPL checkride.  So unless the applicant it disqualified from a medical certificate, an SPL makes no sense in this case.


text:
How do I find the floor of controlled airspace on sectional charts?
So, I understand finding floors and such when seeing something a fraction like 45/10 or 39/SFC, but I've been reviewing and gotten a couple of questions that look like this:
What is he floor of the controlled airspace overlying the Coeur D'Alene Airport?
The answer is "At the surface" but I can't figure out where it says that?
The second question said this:
What is the floor of the controlled airspace overlying the Sandpoint Airport?
The answer is "700 feet MSL". Cant find that either.

It does not say it anywhere because the answer is not variable like class B airspace but lets dig in.
The question here is really trying to get you to identify the airspace that surround Coeur D'Alene Airport and we can see the dashed magenta line encircling it. If you take a look at the legend you will see that denotes

Class E (sfc) Airspace.

The same goes for Sandpoint Airport which is surrounded by a faded magenta line. The legend again denotes

Class E Airspace with a floor 700ft. above the surface

The controlled airspace around Sandpoint begins at 700 ft AGL.  The elevation of Sandpoint Airport is 2131 ft MSL as indicated by the airport data.  The floor of controlled airspace needs to be in terms of MSL which would be the airport elevation + the start of the Class E airspace or 2131 ft MSL + 700 ft AGL = 2831 ft MSL.
I don't understand why the answer would be 700 ft MSL.

In the example figure 22:  Coeur D'Alene Airport is the thick Magenta lined airspace.
(thick Magenta according to legend = airspace with floor 700ft above surface.)
Brooks airport ( dashed magenta)  is "overlying" Coeur D'Alene Airport.  According to the legend dashed magenta = "Class E surface airspace"
what confuses people here is that you also have the VOA (hashed blue line circle/compass rose) overlying the class e airspaces.
the correct answer is "at the surface"  because dashed magenta airspace overlying Coer D'Alene is Class E surface airspace. Surface to 700 AGL

Welcome to AviationStackExchange. As a correction, Brooks Seaplane Base (S76) airspace does not overlie COE airspace.  S76 is depicted by an anchor at the very edge of the Class E to the surface airspace of COE on the shore of Coeur D'Alene Lake at the edge of the city. It does not have its own airspace per se. It is barely in Class E with its takeoff and landing run appearing to be in to be Class G up to 700 feet AGL. COE on the other hand has a Class E to the surface (the magenta segmented circle with an extension) which is then surrounded by a much larger Class E starting at 700 feet AGL.

Also, what is "VOA"?


text:
Does the headwind component change noticeably when transitioning from a crab to a slip on short final?
I don't know if I'm overthinking this, but it has been causing a bit of confusion in my head.
Knowing that the headwind component affects the angle of climb/descent, I was wondering how it affects landing when transitioning from a crab to slip technique?
During a crab the nose is facing more into the wind, but when kicking the nose out to align with the runway, I am trying to imagine if that causes a slight loss of lift or a sudden decrease in descent angle? As there would be more of a crosswind component in relation to the direction the wings are facing?
I might be missing something here, but it seems important to know the physics of this when you're only 100 or 200ft above the runway!

What he said... you shouldn't really start the side slip until you are starting to flare.  The application of rudder, dropping the wing, and flare should be one coordinated motion.  The side slip increases the sink rate, but at this point you are in ground effect and pulling pitch so the effect isn't noticeable.

Overthinking is in the eye of the beholder-- if you are curious about the underlying dynamics, more power to you.

It seems the question has changed significantly since I last answered.  Is the headwind component defined relative to the direction of the flight path over the ground, or relative to the aircraft heading?  It would seem to be the latter.  If the former, the headwind component stays constant (at least momentarily-- not long term) after you "kick out the crab". If the latter, the headwind component decreases, and the cross-wind component increases, when you "kick out the crab".  At the first instant this is true regardless of whether or not you lower the upwind wind to stabilize the situation, while in the long run the decreased headwind and increased crosswind can  only be sustained if you lower the upwind wing to stabilize the situation.  Either way, at the very least you will see an increased sink rate, and in most GA aircraft, a tendency to roll in the "downwind" direction if the ailerons are neutralized, immediately after you "kick out the crab".  This is due to the fact that after you "kick out the crab", the relative wind is coming more from the side and less from the front, in the aircraft's reference frame.  This creates drag and also creates a roll torque due to aerodynamic coupling between slip and roll.

Once again I'll say my personal preference is to eliminate the crab angle and establish the wing-down angle with plenty of altitude-- I'm guessing typically 200 to 400 feet up-- and hold it all the way to rhe ground, adjusting as needed for differences in windspeed and wind direction with altitude.  This is in the context of GA light aircraft.

You know what, the second you talked about ground track it clicked! The heading isn't all that's at play here, such a simple concept I don't know how I missed it! Thanks..

Let's start by looking at a pure "kick out the crab" technique, where we don't lower the upwind wingtip at all.
Even though we don't usually use the word "sideslip" when we talk about "kicking out the crab", we do in fact cause a sideslip when we press that downwind rudder pedal, even if we don't lower the upwind wing.
When you kick out the crab you force the aircraft to slip (or we could say skid) sideways through the air.  A yaw string on the front of the windscreen would blow sideways, in the downwind direction (if there were no propwash).  This is not an efficient way to fly.  Initially, the slip angle is fully as large as the crab angle you just kicked out.  As the flight path starts to curve (because you didn't lower one wing to stabilize the situation)-- i.e. as the aircraft starts to "accumulate drift" (which in reality is actually a turn -- a curve in the flight path--a rudder-commanded "flat turn" or skidding turn)-- things change, but that's the initial situation.  So the more crab you kick out, the more the aircraft is forced to fly sideways.
For any given approach airspeed and any given crosswind component, as headwind increases, so will the required crab angle, so when you "kick out the crab", you'll have more sideways flow over the aircraft, and the sink rate will increase.  You may also notice an increased tendency to roll toward the downwind wingtip, due to slip-roll coupling.
It may help clarify things to understand that "kick" is a little bit of a misnomer.  After the "kick", if touchdown is delayed, to keep the plane aligned with the runway, you have to keep HOLDING some rudder pressure.  If you released it, the plane would tend to swing back into line with the original flight path-- the nose would swing back toward the original heading.  After a few seconds though, the plane has "accumulated enough drift"-- i.e. the flight path has TURNED or curved enough-- that the aircraft heading will be aligned with the runway even with no rudder pressure. You do not want to be in this situation, because it means you have TURNED your flight path through the airmass so much that it is fully aligned with the runway heading, and you have TURNED your ground track toward the downwind edge of the runway.  You'll soon be in the weeds.  That's the trouble with the pure "kick out the crab" technique-- if touchdown is delayed, you have a problem.
Up till now this answer addressed a pure "kick out the crab" technique, but you did in fact mention sideslip in your question.  If at the same time you give the downwind rudder input, you also lower the upwind wing, you'll have the same initial slip angle as described above, and a headwind will affect the required slip angle-- and the resulting sink rate-- in the same way as described above.  But if you lower the upwind wing to stop the rudder-commanded turn, then the situation will be stabilized and you'll see that same slip angle (and that same sink rate) all the way to the ground -- assuming of course that the crosswind component and the headwind component are both constant all the way to the ground, which may very well not be true in reality.
(We're ignoring here the effect that banking has on sink rate, which is real but but probably quite a bit smaller than the effect of flying sideways through the air (slipping/skidding) due to the rudder deflection.)
At any rate, whichever technique you use, for a given crosswind component and a given approach speed, more headwind = a larger crosswind component = a larger slip angle = a higher sink rate.
If you are crabbing along on final approach with NO wing-down correction, and you haven't yet "kicked out" any of the crab angle, then the plane doesn't "know" anything about the wind at all, for a given power setting and angle-of-attack.  It's only after you "kick out the crab", or after you initiate a wing-down cross-controlled correction, that the strength of the headwind matters, because of the way you must adjust your control inputs to match the required slip angle, which is affected by the strength of the headwind.
In actual practice, with the cross-controlled wing-down technique, it seems to work well to imagine that the lowered upwind wing is somehow "opposing the wind" and "stopping the downwind drift", while the rudder is controlling the heading.  The real truth though is that what the lowered upwind wing is really doing, is stopping the rudder-commanded flat (skidding) turn that would otherwise take place if we "kicked out the crab" and didn't touch down immediately after.

PS my preference is to establish a wing-down cross-controlled slip early-- at least a hundred feet up, maybe double that or more-- and hold it all the way to touchdown.  I like the practice. This is in the context of GA.

Thanks for that, love the detailed answer that did help me visualise what's going on better.
I was just curious if there was more headwind component in a Crab, compared to a Slip (With lowered wing).
I suppose, trying to imagine the relative airflow over the wings and how that is affected between the two techniques, but I think the action of lowering the wing must counteract that, because I haven't noticed any major change in practice..

If you want to compare headwind component in a crab (with no "kicking out" yet initiated) versus in wing-down slip that is being conducted purely for the purpose of aligning the nose with the runway-- well-- the aircraft are pointing in two different directions but have the same ground track.  Do we define headwind component relative to ground track or a/c heading?  Whichever way we answer, defines which situation has the greater headwind component.  But I think my official answer above was actually a better fit to your actual original question!

Coming into land in a cross wind seemed, in training, to have 2 schools of thought, drop your wing and apply opposite rudder about half way through final approach, or after rounding out (before flare).  Never liked to even think about doing it while rounding out, as not hitting the ground with my nose wheel had top priority.
One thing to remember is that rudder one way and ailerons the other is cross controlled.  Luckily for me, the Cessna 172 is incredibly tolerant of this.  I practiced cross controlled stalls at altitude with nothing significantly unusual about them.  But I liked to hold the Crab all the way down past round out as it was more coordinated.  It just made more sense (in a small light plane) to crab it all the way in.
Once rounded out and floating over the runway (in lower winds than aloft) rudder straight/wing down, flare, and land.  Wing down is important as it "holds" the plane from being pushed sideways by the cross wind component.
Now, as far as how much airspeed you lose in the "kick", only a fraction from the cross wind.  For example 20 knot cross wind from 25 degrees off nose 65 knots total airspeed.  If it were a 20 knot headwind and was suddenly taken away (highly unlikely), your airspeed would still be 45 knots. However deflecting your nose 25 degrees, your headwind component is cosine 25 degrees x 20 knots = 18 knots!  Your airspeed is now 63 knots, your vertical lift slightly less. So if it is a little windy, carry 5 extra knots in and you're in good shape. The remaining headwind component brings ground speed down considerably.

Possible improvement to this answer: clarify does "rudder straight" mean "straighten the rudder" or "use the rudder to align the nose of the plane (a/c longitudinal axis) straight with the runway centerline".  A slight change in language may make it much more clear. .

It would mean using rudder to align nose with runway center line (while applying aileron into wind).  I forgot to mention it should be held while plane slows to a stop to help control "weathervaning".


text:
Why is Transition Altitude fixed but Transition Level is not?
In Jeppesen approach plates we see that the Transition Altitude of the aerodrome is fixed but the Transition Level is not fixed and is given by ATC. What is the reason for this?

Related: https://aviation.stackexchange.com/questions/82260/can-transition-altitude-ta-and-transition-level-tl-be-equal/

The point of flying on QNH is to keep you from flying into terrain, which is at a constant altitude above sea level, so TA is fixed.
Once you are high enough that terrain can’t be a problem and you only have to worry about hitting other planes, everyone can transition to QNE. However, because TL must be physically above TA, the difference between QNH and QNE will dictate what the lowest usable flight level is.

you might like to add this factoid: In some countries like the USA, they've addressed the issue of a different TLvl by having the TAlt and the TLvl = 18,000ft and FL180 respectively by choosing this value of TAlt so that it clears all terrain in the area of applicability.

@skipper44 The FAA claims their TL is always FL180, but then they sometimes declare the lowest FLs to be “unusable” depending on QNH, which in practice is no different from having a variable TL.

thanks for the elaboration. Here's more on lowest usable FL


text:
Does Ryanair lease any aircraft?
Does Ryanair lease any of its aircraft?
And if they do, could the end of the lease period coincide with the start of the low passenger periods? E.g. during the autumn to spring months when holiday traffic is slow, there may be a need to reduce the fleet size.

Welcome to Stack Overflow Aviation, could you please reformulate your question so we can understand better what you are looking for ?

@PierreChevallier Seems pretty clear to me. Do Ryanair lease their aircraft? Do they arrange for the lease periods to end at times when there's less traffic?

It's hard to know what the real financial status of a purchased aircraft is. It is common for larger carriers such as Continental (who I use to work for) to report 'they' were purchasing 20ea B757 when each aircraft was in fact purchased and owned by investment groups on a lease back. Continental projected themselves as the owners of their fleet but in fact had negotiated the deal in bulk for third parties. The airline business is all smoke and mirrors - interesting question though. I am sure for the purposes of the question, if Ryanair says 'they' own something that is good enough.

Ryanair mainly buys their aircraft. Part of their ability to offer cheap flights comes from buying aircraft in bulk and generally only operating a single type of airframe.
They generally buy new Boeing aircraft operating primaraly the 737 series.

...had agreed to buy five more Boeing 737-800 aircraft, taking its order
  book to 180 new Boeing planes worth over $16 billion.

Planespotters.net has a nice list of the aircraft in Ryanair's fleet, with the exception of this -700 the rest of the aircraft only show registration to Ryanair as an owner under owner history.


text:
Are these logged time assumptions correct for commercial checkride applicants?
If an applicant is an ASEL private pilot without an instrument rating, are these assumptions correct?

Multi-engine dual time can be used to satisfy the 10 hours complex time requirement
Cross-country time flown as a solo student pilot can be used to satisfy some of the 50 hours total cross-country required, including the long (300 mile) cross-country flight
Cross-country time flown as multi-engine instruction cannot be used as cross-country PIC because the pilot was not rated in category and class

Welcome to aviation.SE! I reworded your question a little for clarity but if I got something wrong then please do edit it again as necessary. I also assumed you're asking about the US because you used the term ASEL; when asking about regulations or procedures, please always tell us which country you're asking about.

@Pondlife ASEL is not US-specific terminology

@DanHulme Thanks, I didn't know if it was or not!

Your question is slightly unclear, but I'm assuming the following:

You're asking about the US/FAA Part 61 requirements
The candidate is planning to take the CSEL checkride
The candidate holds a private ASEL certificate, without an instrument rating
The candidate has received some MEL training (you didn't say how much), but isn't yet rated for MEL

In that case, the answers are:

61.129(a)(3)(ii) requires 10hrs of instruction in a complex or turbine aircraft, but it doesn't say how many engines the aircraft has to have or what the training has to include. So using time from general MEL training is fine.
The FAA has said that 'double-dipping' time from private to commercial is not allowed, therefore flight time as a solo student cannot count towards the commercial certificate (Theriault (2010) interpretation).
If the pilot was receiving instruction in an aircraft that they weren't rated for then there's no way for them to log PIC time because none of the scenarios in 61.51(e) for logging PIC time apply. As you said, logging PIC as sole manipulator of the controls also requires you to be rated in the aircraft.

(This question is related.)


text:
Has anyone built a paragliding simulator?
I am searching for a manual on how to build a paragliding simulator, where you can practice controlling the paraglider. My idea of the project is that you sit in a paraglider seat and you can experience the behaviour of the seat in a flight.
If you have any references to already built projects, it would be awesome.

You mean this sort of thing? http://xcmag.com/news/new-paraglider-flight-simulator/

Ubisoft released "Steep", a mountain sports game that include paragliding. But it wasn't realistic at all and disappointing IMHO.

The popular simulator X-Plane allows you to build your own aircraft models it looks like some people have built paraglider models that you may want to take a look at. As for controls you can build all kinds of custom interfaces to simulate the controls of a paraglider but it will take some work.

For the sake of completeness, you may add that custom output interfaces would need to be employed, in order to experience the behavior of the seat. (Tilting the seat, and the image, etc).

Hey Dave thank your very much for your good suggestion. Do you know somehting about an real life steering seat. Like what they build here, I am searching right now for similar projects like this: http://www.usppa.org/schools-training/initial-training-for-new-paramotor-pilots/powered-paragliding-simulator/

Here is an operational paragliding simulator (located in Amsterdam, Netherlands) that may serve as an example: https://flightcoach.nl/en/services/vrsim/
It did not yet exist when the question was asked.

Welcome to AvSE!

Yes. The game Paragliding Sim is currently on version 4.1. It is available on Windows, Linux and Mac.

The most realistic paragliding sim ever made!
The wings are softbody, meaning they can flex and twist!
Fly cross coutry, ridge soar or speed fly the terrain with any glider
of your choice. This simulator includes dynamic wind (ridge lift, lee
side rotor) as well as thermals which are most likely found on the
sunny faces of the terrain.
Recently updated with anabatic (sunny slope) winds!
Now has paramotors!
In the future I plan to add:

Hang Gliders
Fixed Wing Aircraft

The stated question isn't about a simulator on a computer screen, but using actual hardware; I don't believe this post actually answers the question as asked.

Fair enough. But I think it would be better than the alternatives if you build a hardware to go with it.


text:
How are small airplanes steered on the ground if they don't have tillers?
I have noticed that many small planes do not have tillers like in big aircraft.  So how does the pilot steer them?

Most small planes (there are exceptions!) have three wheels - two fixed wheels, one under each wing, and one under either the nose or the tail.
When the third wheel is under the nose ("tricycle" layout), that wheel is usually (but not always) steerable with the same pedals as the rudder, enabling accurate steering on the ground.

(Image Source: AvWeb - Author: Joseph Burnside)
The rudder itself will also add an element of steering moment depending on the airflow over the fuselage (taxiing speed is usually too low for this to have much effect, except that on single engine aircraft applying power will push air over the fuselage and tail, adding effectiveness to the rudder).
In addition, for sharp turning at low speeds, differential braking can be applied - press the brake pedal for the starboard main wheel only, and the plane will tend to pivot about that wheel - ie turn right.
When the third wheel is under the tail ("tail-dragger" layout), the tail wheel is not usually steerable (it usually pivots freely - but again there are exceptions!). In this case the other two techniques mentioned above - rudder and differential braking - are the only methods of steering on the ground. I've never flown a taildragger myself but I understand the taxiing technique takes some getting used to.

(Image Source: WikiMedia Commons - Author: RenioLinossi)

if the small aircraft have the engines on the sides, then might be the throttling can also be used for steering but I am not sure.

@NitinG in theory yes, but the braking technique is much more responsive and accurate.

IanF1, The Scheibe Falke SF25 has a steerable tailwheel, as it also comes in this configuration, which doesnt allow other steering.

There are lots of different kinds of tailwheels. Some are free catering, some are steerable, some can be locked straight ahead but caster for turns.

The Cessna SkyCatcher is a tricycle gear plane, but the nose wheel is not steerable via the rudder pedals.  All ground steering is accomplished with the rudder (which has low effectiveness at slow taxi speeds) and with differential braking between the two mains.

Thanks to whoever added the photos! Also I agree that there are exceptions to the "nosewheels steer, tailwheels don't" rule, but am not sure how to edit that in gracefully.

@abelenky: I'm no great expert, but I've never seen a steerable nosewheel on a tricycle-geared light plane.  Instead, the nosewheel casters, and steering is by differential braking of the main wheels.  (In the nosewheel picture above, I think the rod going back to the frame is most likely a vibration damper.)

@jamesqf:  Cessna 172 is definitely nosewheel steering.

@abelenky So are all the Piper PA28 Cherokees I've flown. Wonder if jamesqf meant something different by "light plane"?

The Cessna 150 also uses a steerable nose gear; after lift-off the gear strut extends to its maximum travel which also serves to disconnect it from the rudder linkage.

The PiperSport LSAs, however, do not have steerable nosewheels and use differential braking instead.